The objective of the proposed research is to study the regulation of glomerular filtration rate by testing the hypothesis that filtration rate is plasma flow dependent in the dog, a species characterized by filtration disequilibrium. The experimental approach will be to determine the mechanism by which the gastrointestinal hormones secretin and glucagon increase filtration rate. Recollection micropuncture will be used to measure changes in glomerular plasma flow and transcapillary Starling forces. The ultrafiltration coefficient will be calculated. The quantitative contribution of each of the determinants of filtration will be evaluated by computer analysis of a previously described model for glomerular ultrafiltration. The site of altered renal resistance will be determined by identifying the changes in pre and postglomerular resistance. In addition, the significance of filtration disequilibrium for the regulation of filtration rate will be evaluated by studying the effects of renal vasodilation in a model of filtration equilibrium in the dog. Finally, we will test the hypothesis that the failure of certain vasodilators to increase filtration rate is due to enhanced production of intrarenal prostaglandins which reduce the ultrafiltration coefficient.